The present invention relates generally to a focus monitoring apparatus in an optical recording apparatus, and more particularly to a focus monitoring apparatus capable of monitoring the focused states of two optical beam spots and two optical beams to be focused on a surface of a recording medium with high accuracy.
The systems known heretofore for recording and reproducing information signals such as video signals and/or audio signals on and from disc-shaped, rotary mediums (referred to hereinafter simply as "discs") are broadly divided into sysems in which reproduction is carried out optically, systems in which reproduction is carried out by means of a reproducing stylus employing a piezoelectric element, and systems in which reproduction is carried out by utilizing variations in the electrostatic capacitance between an electrode on the reproducing stylus and the recorded surface of the disc.
We have considered the advantages and disadvantages of these three kinds of systems and have adopted the electrostatic capacitance system as being the most desirable. Further, the present inventor has described a new system for overcoming the problems accompanying known electrostatic capacitance system, in a U.S. patent application entitled "Information signal recording and reproducing system", Ser. No. 785,095.
More specifically, in accordance with a known electrostatic capacitance system, recording is accomplished by forming a spiral guide groove in the disc for guiding the reproducing stylus. At the same time, pits are formed in responsive correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus, being guided by the guide groove, traces the track within the groove and reproduces the signal in response to variations in the electrostatic capacitance.
In this known system, however, a reproducing stylus guiding groove is provided in the disc. The reproducing stylus is compulsorily guided by this guide groove. It is not possible for the reproducing stylus to undergo operations such as riding over the groove wall of one track, moving into another track and returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, both the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out a special reproduction such as quick-motion picture reproduction, slow-motion picture reproduction, and still-picture reproduction.
Furthermore, since the area of contact between the tip of the reproducing stylus and the guide groove of the disc is small, the stylus easily wears. When the width of the stylus tip reaches the groove width as a result of abrasive wear, the serviceable life of this reproducing stylus ends. Thus, this system is accompanied by the problem of a short life of the reproducing stylus. Another problem is that fine chip particles are abraded from the disc by the tracing action of the reproducing stylus along the guide groove. These chips give rise to difficulties a such as further promotion of the wear and damage of the stylus and the guide groove and an increase of the gap between the electrode at the stylus tip and the recorded surface including the pits within the disc guide groove, whereby the reproduced output decreases because of spacing loss.
If the rotational speed of the disc is set at a low value such as 450 rpm., for example, in order to prolong the life of the stylus tip, the recording wavelength of the information signal recorded in the guide groove will become short. For this reason, a recording apparatus of low price, in which laser light is used, cannot be employed in the recording system, and the disadvantageous employing of a high price recording apparatus using an electron beam becomes unavoidable.
Accordingly, with the aim of overcoming the various problems of the known systems described above, the present inventor has described in the above cited patent application a novel system for recording and reproducing information signals. According to this application, in the recording system, pits are formed in accordance with the information signal being recorded along a spiral track on a recording medium of a flat disc shape, without forming a groove therein. In the reproducing system, a reproducing stylus traces over and along this track to reproduce the recorded information signal. In this proposed system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal. A tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this previously described system, the recording track has no groove. There is no possibility whatsoever of the reproducing stylus or the recording medium being damaged. The stylus can repeatedly trace the same portion of the track repeatedly a plurality of times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, other difficulties of the known system are removed.
However, in the above described system, it is necessary to record the main information signal on the main track. The reference signal for tracking servo control is recorded on the sub-track of the recording medium.
Hence, it becomes necessary to perform the recording not to leave space (guard band) between the tracks, in order to increase the recording capacity on the surface of the recording medium. Accordingly, the adjacent main tracks are in closed contact with each other. The sub-tracks are formed at a center position between the center lines of the main track, and overlapping the edge parts of the adjacent main tracks.
Therefore, the positions of the spots formed by focusing the main optical beam for forming main tracks (for the main information signal) and the sub-optical beam for forming sub-tracks (for the reference signal) on the surface of the recording medium, are extremely close to each other. When the track pitch of the main track is, for example, 1.4 .mu.m, the distance between the center lines of the main track pitch and sub-track is 0.1 .mu.m.
In the above case, to prevent the generation of beat interference due to the cross-talk between the adjacent tracks upon reproduction of the recording medium, the error in the distance between the above two optical beam spots must be less than 0.1 .mu.m.
Accordingly, before starting the recording on the recording medium, it becomes necessary to adjust a focus so that the above two optical beams are focused normally. Normal beam spots are formed. Furthermore, it is necessary to adjust the two beams so that the relative positions of the two optical beam spots are in the correct positions.
The emitting direction of the laser beam emitted from a laser emitting device which is used as a laser beam source, varies according to the variation in temperature. The length of the respective optical path of each of the above two laser beams, the characteristic of the optical systems, and the like, are not completely equal to each other. Thus, the relationship between the relative positions of the spots of the two laser beams varies according to temperature. Hence, during recording on the recording medium, it is necessary to adjust the spot positions so that the relationship between the relative positions of the two laser beam spots do not shift.
In order to accurately perform the above described adjustment, it is necessary to clearly and accurately monitor the distance between the above two beam spots, the focusing of each of the beam spots, and the like.
However, in the case where the allowable value for error in the distance between the beam spots is small, as compared to the diameter of the beam spot, the distance between the beam spots cannot be accurately monitored or measured by providing a monitoring optical system which simply enlarges and images the reflected beam from the beam spot on the surface of the recording medium, since the contour of the beam spots, which are monitored, blurs, and since the resolving power of the monitoring optical system is low. This will be explained in the following specification along with a numerical example.
When a laser beam is employed having, for example, an objective lens having a numerical aperture (N.A.) of 0.9 and wavelength of 4579 .ANG., the half-value of the diameter of the optical beam spot focused and formed by the objective lens is approximately 0.3 .mu.m. When the error due to the variation in the distance between the two beam spots is less than 0.1 .mu.m, as stated above, even if the beam spots are enlarged and monitored, it is very difficult to measure and monitor the relationship between the relative positions of the two beam spots, with a high resolving power. Particularly, when a slit plate is located immediately in front of the objective lens, to accurately form the beam spots into a desired form, the substantial numerical aperture (substantial numerical aperture in the radial direction of the recording medium) is reduced to approximately 0.45 to 0.5, and hence the resolving power decreases. In the above case, the diameter of the beam spot becomes larger than the above 0.3 .mu.m, and hence the monitoring becomes even more difficult.
Accordingly, an apparatus capable of accurately monitoring the beam spots has been highly desired.